The Synodontis I can only seem to find fully grown fish and as I will probably be getting F1 juvenile cichlids I dont want these turning into rather expensive catfish food

The B/n cats eat the algae that my mbuna will also want to eat

This leaves the Zebra Loach I have noticed quite a few members keeping loaches of some sort or other with their Malawi cichlids they wont strip the tank bare of algae or hunt down my little F1 cichlids

So was thinking of getting 3 Zebra Loach and single B/n cat does that seem ok for a 48x18x18 set up?

Hmmm - me I would go with the synodontis, but then I have a soft spot for them. Bristle noses do well and a lot of people have them, not sure about loaches as I would have thought they would do better in a softer water set up ?

There are a lot of synodontis and most would be ok .... I have kept a syno ocelifer with malawi's for about 20 years, he nabs a few fry but some make it.

I wouldn't worry about the synos gobbling your cichlids when you get them. Yes, they will make a meal of the odd tiny fry i.e. when they are first spit, but presumably when you say you will be stocking with fry I assume you mean something at least guppy-size. Synodontis lucipinnis (aka dwarf petricola which I assume they really are) won't take young fish of that size.